This invention relates to a welded structure capable of having its mechanical strength, such as brittle rupture strength, fatigue strength, etc., improved by reducing welding residual stress by means of stress relief annealing treatment, a welded structure having its mechanical strength improved by the aforesaid treatment, and a process for making same.
In welded structures, particularly welded steel structures, tensile residual stress tends to be produced in butt welds, T-joints and fillet welds after welding. It is usual practice to remove or alleviate such residual stress by subjecting the welded structures to stress relief annealing treatment.
The stress relief annealing treatment is carried out in order to improve the strength of a welded structure by alleviating tensile stress and improving the microstructure of the welding heat affected zone. The welded structure is usually kept at a temperature tending to cause transformation of steel from .alpha. to .gamma. and vice versa (preferably in the range between 600.degree. and 650.degree. C.) for about one (1) hour per one (1) inch of the thickness of the welded structure, and then slowly cooled to about 300.degree. C. or below that level (particularly at a cooling rate of less than 50.degree. C. per hour and preferably by furnace cooling). Materials capable of having their residual stresses relieved by annealing include carbon steel, low alloy steel and stainless steel (high alloy steel).
In joining structural members by welding, it is common practice to deposit a weld metal of the same type as the metal of the structural members to provide a defect-free weld to the welded structure. Thus the structural members to be welded and the weld metal used in welding have substantially the same coefficient of thermal expansion. Because of this, when the weld (including the welding heat affected zone) of the welded structure is subjected to stress relief annealing treatment, residual tensile stress would develop on the surface of the weld metal although such residual stress is less than 10 kg/mm.sup.2 in magnitude. The mechanism of development of such residual tensile stress would be that the welded structure including the weld would undergo thermal expansion when it is heated and tensile stress would remain on the surface of the weld metal which is restrained by the welded portion of the welded structure when the latter is cooled. The tensile stress that remains would, of course, be less than one third to one fourth in magnitude of the tensile stress which would be present on the surface of the weld metal when no stress relief annealing treatment is performed. Nevertheless, there is the risk that brittle fracture and fatigue fracture would occur in the weld that has been subjected to stress relief annealing treatment, and such weld would be susceptible to stress corrosion cracking while in service.